Probably trivial, but I'm a programmer - not a ee

oops!

all diodes should be pointing to the right,

ls 4 servos which actuate 4 mallets on 4 chimes to play a melody. I have t he arduino and servos operating as expected, interfacing a standard doorbel l button is the trick I just can't seem to figure out.

orbell you might buy at home depot. This means I have 5 wires to interface (7 if using a button for back door chime). 3 wires for AC power, 2 wires per doorbell button.

ng to a non-lighted button is trivial. It's just a regular switch and I ca n push 5v to capture that input. Lighted buttons normally run on 12vac, bu t will also light on 12vdc. Unfortunately, the bulb is wired in parallel w ith the button - so there's constant current. Additionally, the arduino ca n't take 12v on the inputs - so there's no way for me to use a lighted butt on without some magic.

igure the lighted button with a resistor and LED and run the whole thing on 3.3v from the arduino. The reason I don't want to do this is because - as a gift - I want this to be as transparent to the recipient as possible. I f at some point the button needs to be replaced, the new button would need to be made compatible.

to the same source - pressing the button drops voltage to the arduino - ca using it to reset. So the button circuit needs to be isolated from the ard uino.

a 5vdc device - what would you do?

I you want a "drop-in" replacement, it will REQUIRE the switch and doorbell transformer remain untouched. So you have three wires to contend with: tra nsformer common (COM), front door switch output (F), and rear door switch w ire (R), that's it. The most common doorbell transformer voltage out there is 16VAC. What you're doing now is way out of line with drop-in. Either fig ue out how to use those three wires, or move on to another project.

rols 4 servos which actuate 4 mallets on 4 chimes to play a melody. I have the arduino and servos operating as expected, interfacing a standard doorb ell button is the trick I just can't seem to figure out.

doorbell you might buy at home depot. This means I have 5 wires to interfa ce (7 if using a button for back door chime). 3 wires for AC power, 2 wire s per doorbell button.

cing to a non-lighted button is trivial. It's just a regular switch and I can push 5v to capture that input. Lighted buttons normally run on 12vac, but will also light on 12vdc. Unfortunately, the bulb is wired in parallel with the button - so there's constant current. Additionally, the arduino can't take 12v on the inputs - so there's no way for me to use a lighted bu tton without some magic.

nfigure the lighted button with a resistor and LED and run the whole thing on 3.3v from the arduino. The reason I don't want to do this is because - as a gift - I want this to be as transparent to the recipient as possible. If at some point the button needs to be replaced, the new button would nee d to be made compatible.

ed to the same source - pressing the button drops voltage to the arduino - causing it to reset. So the button circuit needs to be isolated from the a rduino.

to a 5vdc device - what would you do?

ll transformer remain untouched. So you have three wires to contend with: t ransformer common (COM), front door switch output (F), and rear door switch wire (R), that's it. The most common doorbell transformer voltage out ther e is 16VAC. What you're doing now is way out of line with drop-in. Either f igue out how to use those three wires, or move on to another project.

What is your f'ing problem you can't wake-up the HOLY Arduino when F or R g oes live? The real question is what could be simpler. That crummy little il lumination current on the buttons is not an issue, easily handled, unless y our brain s fossilized like a certain somebody living on welfare water in A z.

I did a little digging around and found a web page where someone messed with his doorbell. He has enough numbers for me to realize that the difference between the light current and the bell current is enough that the light current will not drop an appreciable voltage through the transformer or the doorbell. Seems they draw around 2 amps when you push the button! I had no idea... What current is your 12 volt supply rated for? Oh, 2 amps should be good then.

Interesting that this web page was about replacing the lamp with an LED. Also, while I was searching I found lighted doorbell buttons *with* LEDs in them. That wouldn't do you much good since they include a series resistor to limit the current and you would need to have that resistor external to the button so just the voltage on the LED is accessible for the LED to be any advantage for you.

Here is the link. Notice his lamp current was around 75 mA and the LED just 3 mA.

Good point. Those doorbell switches may be cheap, but they typically can handle 2A or more, BUT that's _AC_ not DC. He's using 12V DC, so it might work just once. At the first push of the doorbell button, the points might arc and weld when the button is released. Even if it works quite a number of times, the points in the button will be subjected to more pitting than if the circuit used AC, and the button will become defective sooner with DC (likely far sooner) than with AC.

My doorbell measures 7.6 ohms (DC), and it's rated at 16V 10VA, so we can approximate the AC impedance of the doorbell as ~18 ohms. That, plus its DC resistance equals 25.6 ohms, which agrees with your thinking about total doorbell impedance.

I would think the same thing which is why I said: "If it glows to your satisfaction, you're done. If not, read on." The majority of my post applied only if the OP thinks the bulb does not glow to his satisfaction.

Well it really looks like it will maintain regulation. If the figures are right (12V 24 mA) the bulb will draw well over the minimum current to keep the 317 in regulation. The bulb exhibits about 500 ohms when hot (12/500 = 24mA) and lower resistance when colder at lower voltage. Even if we assume 500 ohms at 9v, it'll draw about 18 mA. The maximum current needed to keep the LM317 regulating is 10 mA.

As to the drop out spec of 1.5 volts, there is an additional drop of 1.25v across the resistor from Vout to adj. So, at

Yes. The beauty of the LM317 circuit is that, with current limited to 24 mA, the push button will likely last a long long time. :-)

Well, I don't know if we are being overly cautious. His

Ed

But if the bulb draws 24 mA at 12 volts and the circuit is designed to deliver 24 mA, the bulb won't be drawing 24 mA at the lower voltage after the regulator, so the regulator will be turned on as hard as possible with no ability to regulate. Still, that's fine. It will have less than the rated drop out voltage on it (giving the bulb closer to its full 12 volts) and when the button is pushed the current will stop increasing at 24 mA.

That's where I got it.

Once I looked at the data sheet and saw this was a multi amp part in a TO220 package I realized it didn't blow up because it couldn't handle the 24 mA or whatever was put through it. Something was wrong with the wiring or possibly the part was defective or maybe the wrong part entirely. You read about counterfeit parts and I'm always leery of buying semis from eBay and such sources (Alibaba has to be the worst). Digikey and similar should be ok as they buy direct from the manufacturers.

I suppose that depends on how you look at it. The purpose of the circuit is not to regulate the current through the bulb. For its purpose it retains the ability to regulate. As soon as the circuit attempts to draw more than 24 mA, the 317 lowers the voltage at its output pin. The whole point of the circuit is to limit current when the button is pressed, and it will do that regardless of dropout voltage in this application. Dropout voltage can affect the brightness of the bulb, but not the required limiting when the button is pressed.

Right.

Yes - once burned, twice shy.

I am slightly puzzled by the choice of circuit. What prevents you from sensing the line that goes off to the switch and lamp with a simple resistor and 5.1v zener diode to clamp the voltage the CPU line sees?

com:

worked perfectly. I'm still experimenting with resistors to get the curre nt to the button down, and to achieve the correct voltage out for the ardui no - but it's more success than I had on my own. The arduino doesn't reset on button press, and as described - voltage drops to zero when the button is pressed.

a piece flew in her direction and she nearly jumped out of her skin. "Are you trying to burn the house down??" she yells.

experimenting to determine the minimum current necessary to keep the bulb lit (and then maybe raise it a little higher to accommodate power fluctuati ons). Apparently I started too low and/or held the button too long. My ea rlier tests, I only held the button for less than a second.

id future pyrotechnics? Am I mistaken as to the cause - do I need a more r obust heat sink, was it heat related? I'm thinking my next attempt will be at 240ohm, but I don't want to get yelled at twice for the same thing.

The button shorts the supply you need something to limit the current

-Lasse

Adding a 39R 5W series resistor to the PSU feed ought to do it then.

Surely the button completes the high current circuit so that the bell solenoid pulls in. That is how most of these ding dong bells work.

Back EMF from the bell solenoid may well kill the unprotected 317 too.